Did Pollution Drive Fish's Evolution?

April 17

By Lee Dye

An unpretentious little fish that has learned how to thrive in one of the most polluted rivers in North America has scientists wondering if foul water is driving its evolution and, ironically, whether genetic changes that seem to be taking place might jeopardize its ability to survive in cleaner water.

The object of all this attention is a minnow-like fish normally used for bait, called the Fundulus heteroclitus, or more commonly, the killifish. Along the waters it inhabits in Virginia's notorious Elizabeth River, it's known as the mummichog.

It's not that anyone is losing any sleep over the future of this little fish, which measures a mere 2 to 3 inches long at maturity. But the mummichog could tell us much about the long range impact that human activities may be having on other species.

"We're interested in how human disturbances affect other organisms over multiple generations," says Richard T. Di Giulio, an environmental toxicologist at Duke University, who is leading a study sponsored by the Office of Naval Research.

If it could talk, the mummichog would have a heck of a story to tell. Di Giulio believes at least 50 generations have survived in the foul waters of the Elizabeth River, adapting to water so polluted that one study calls it a "toxic hot spot." Indeed, if the riverbed is disturbed, oil normally gurgles to the surface, creating a slick.

Clean Water Casualties

But when Di Giulio captures the fish and takes them to his lab to offer them a new home in a pristine aquarium, many of them die.

"They survive poorly, and their offspring don't survive well either," he says. Fish taken from clean water have no trouble adapting to Di Giulio's aquarium.

Fortunately, some of the fish taken from the Elizabeth do survive. They reproduce in the aquarium about six weeks after being taken from the river, and some of their offspring survive and reproduce as well. That gives Di Giulio three generations to study, and the results so far are disturbing and inconclusive.

Vulnerable to Viruses

The mummichog paid a price for their adaptation to the murky waters of the Elizabeth. Adult specimens look healthy, but they have liver cancer. That was expected because of the high concentration of carcinogens in their home waters.

But there's something more perplexing. They also seem to have lost their ability to protect themselves from some bacteria and such things as fungal diseases.

Why should fish that adapted to very bad water have more difficulty surviving in clean water?

It's little more than a hunch at this point, Di Giulio says, but it may be that the pollution in the river killed the bacteria that causes such diseases, lessening the workload of the mummichog's immune system. Over time, he speculates, the mummichog's immune system may have become partially suppressed because some diseases it would have fought are no longer a threat, at least as long as it remained in the polluted waters.

So when they are taken to the lab, they aren't properly equipped for more normal conditions.

"They are more prone to various infections, and that's really why they are dying in the lab," he says. "They are dying of various fungal and perhaps bacterial infections."

A Case of Genetic Manipulation?

Enough have survived, however, to give Di Giulio a chance to determine whether the effort to adapt to the foul waters of the Elizabeth River actually led to evolutionary changes. In other words, did pollution drive evolution?